This invention relates to improved forming fabrics for papermaking.
It has long been known that to provide maximum resistance to abrasion in forming fabrics, large wear resistant yarns must be used. However, when such yarns are used in single layer woven fabrics, the resultant meshes are not fine enough to prevent the loss of fines through the fabric and paper quality suffers in terms of wire mark and noticeable sheet two sidedness.
In the case of single layer woven fabrics for papermaking, a delicate balance must be made between making the fabric mesh too fine with consequent reduced wear life, and making the fabric too coarse at the expense of sheet quality.
To overcome these drawbacks, there has been an ever increasing trend in recent years toward replacement of single layer forming fabrics with multiple layer forming fabrics. Such multiple layer woven forming fabrics may be woven with fine mesh in the sheet contact side and coarse mesh in the wear side of the fabric.
While providing at least a partial solution to the operational drawbacks of single layer woven forming fabrics, the multi layer forming fabrics are much more costly and time consuming to manufacture since they invariably contain finer yarns and more of them than in the equivalent single layer fabrics. In some cases, new machinery may be needed to weave the more sophisticated multi layer fabrics. In fact, at almost every step in the manufacturing sequence, the process is slowed down because of the higher number of yarns involved and the need for greater attention to detail to avoid fabric damages.
This trend toward more intricate weaves and multi layer fabrics has taken place as a direct result of the need to achieve higher paper quality standards at ever increasing paper machine speeds.
Multi layer fabrics enable the fabric designer to provide the papermaker with a fabric which can be designed for improved sheet support and fines retention in the sheet contact side, and on the machine contact side, for long wear life.
At the start of this trend, double layer forming fabrics led the way. Now, triple layer fabrics are seen as even better in meeting the need where sheet quality is of prime concern.
But in spite of this increased weaving sophistication, a significant percentage of fines are still lost during formation, because it has not been found practical to weave a forming fabric in sufficiently fine a mesh to prevent the escape of the smallest particles that exist in the furnish.
Only after an initial precoat of long fibers forms above or between adjacent strand members does the particle capture efficiency begin to increase. On many forming machines, the sheet comes under such severe agitation during this formation phase that the initial sheet precoat is disrupted repeatedly, causing recurrent losses of fines during much of this early dewatering phase.
During sheet formation, the forming fabric surface must be initially bridged over by particles in the furnish that are at least as long as the space between adjacent fabric elements. Until this happens, pore spaces between yarns serve as ready passageways for egress of small stock particles called fines.
After the initial bridging takes place, it is commonly assumed that the precoat forms up more or less permanently to thenceforth improve the particle capture efficiency of the combination of fabric and precoat; however, recent studies have shown that this is not always the case. Owing to high machine speeds and to disturbances during formation, turbulence takes place, with the result that the initial precoat is disrupted more than once, each time with the loss of more fines until the precoat again becomes established to prevent further loss of fines through the mesh.
The fines that are lost during sheet formation are fed back into the headbox, which increases the fines concentration of the furnish. This constant recycling can reduce the drainage rate for the machine. With regard to such loss of fines, it is known that under some conditions, even small differences in fabric mesh can effect fines retention significantly.
One of the drawbacks of multi layer woven forming fabrics is that particle matter penetrating through the relatively fine mesh of the sheet contact layer frequently lodges between the multiple layers of the fabric. To prevent the buildup of particle matter, intense showers are needed. Paper machines without such showering capability, particularly older, open draw machines, may have extreme difficulty in running these multi layer fabrics without incurring early fillup and plugging of the fabric. Thus, in addition to the premium papermakers must pay for the multi layered fabrics, they frequently have to add expensive auxilliary showering equipment to be able to operate these fabrics.
Perfection of the forming fabric is prerequisite to the production of high quality paper. In the case of endless woven forming fabrics, imperfections in the woven edges can cause sheet breakdown and disruption of operations. In the case of joined forming fabrics, the fabric must be very painstakingly joined into the form of an endless belt in order to operate on the paper machine. Even small discrepancies in the perfection of the fabric join may result in sheet breaks or loss of sheet quality. The presence of even slight imperfections in the weave may be cause for rejection of a very expensive finished forming fabric.
In the field, forming fabrics may be expected to operate satisfactorily for periods of from one to several months--unless they are subjected to some form of accidental damage. Then, efforts are made to restore the fabric to its original characteristics by patching the damaged area. In most cases, this is a time consuming task, and often, it is not possible to make a perfect repair.
Forming fabrics effectively form a paper sheet by capturing paper fibers on their sheet contact surface. They perform this function more or less efficiently based upon the fineness of the mesh. One way of expressing this fineness is the strand to strand distance. In even the finest forming fabrics, woven in multi layer weaves, the strand to strand distance exceeds the length of the fines by a very substantial margin.
Dryer fabrics for paper machines have for many years been supplied as flat belts and joined together after installation on the dryer with the aid of a pin seam. More recently, wet press felts have been furnished in flat form and successfully joined together on the paper machine. Such ease of installation and seaming in place on the paper machine has never been possible for forming fabrics owing to the need for uniformity in the sheet contact surface of the forming fabric.